iio: light: vcnl4000: Preserve conf bits when toggle power

iio: light: vcnl4000: Preserve conf bits when toggle power

As the vcnl4040 and vcnl4200 chip uses runtime power management for
turning the ambient light and proximity sensors on/off, it overwrites
the entire register each time. In ALS_CONF register bit fields ALS_IT,
ALS_PERS, ALS_INT_EN are overwritten. In PS_CONF1 register bit fields
PS_DUTY, PS_PERS, PS_IT, PS_HD, and PS_INT are overwritten.

Add functions for preserving the affected bit fields when changing power
state.

Signed-off-by: Mårten Lindahl <marten.lindahl@axis.com>
Link: https://lore.kernel.org/r/20220926091900.1724105-2-marten.lindahl@axis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>